Cold asphalt paving process



Dec. 31, 1968 u. B. BRAY ET L COLD ASPHALT PAVING PROCESS Filed May 29,1967 ASPHALT LJ M/LL STONE jA/VEIVTORS 1.2/6 5. BRAY (/4 CK E. W000ASPHALT POWDER a DUMP TRUCK Fl UX O/L United States Patent 3,418,900COLD ASPHALT PAVING PROCESS Ulric B. Bray, Pasadena, and Jack E. Wood,Upland, Calif., assignors to Bray-Wood Company, Upland, Calif., apartnership of California Filed May 29, 1967, Ser. No. 642,050 6 Claims.(Cl. 94-23) ABSTRACT OF THE DISCLOSURE A method of paving with asphaltand stone without heating, wherein the stone is first uniformly coatedin a mixing machine with a powdered, hard asphalt, then mixed with aflux oil and subsequently spread onto the pavement where it iscompacted, as by rolling. Excellent distribution of the asphalt and fluxon the stone is obtained in a minimum time of mixing.

This invention relates to a process of paving with asphalt and stone,applicable to roadways, parking areas, tennis courts and other playingfields, revetments and to any suitation where the familiar hot mixasphalt is now used. One object of the invention is to avoid the need ofheating the stone aggregate and spreading it before it becomes cold asin the usual hot mix process. Another object of the invention is toprovide a simple, low cost portable machine for preparing asphalt pavingmix which can be located near the site of the paving project, therebyreducing the travel time for transporting the mix to the job. Stillanother object of the invention is to provide a method of mixing asphaltand stone aggregate accurately and rapidly to give a uniform productmeeting all highway specifications without the need of stock pilin andrehandling.

Our invention employs the principle of blending hard asphalt in the formof a fine powder with a flux oil and stone aggregate in the cold, thatis, at temperatures prevailing at the job site, whereupon the powdered,hard asphalt and the flux oil, more or less slowly, depending ontemperature, particle size, etc., combine or coalesce into a strong,adherent binder to hold the stone together in the pavement. Thisprinciple has been the subject of numerous attempts heretofore tocapitalize on this inherent advantage of a cold asphalt paving processas shown by such patents as Amies, 945,071; Radcliffe, 1,655,240;Beckwith, 2,220,670; and Mollring, 2,785,163.

However, the process has been beset with numerous difiiculties viz: (1)moisture in the stone has interfered with mixing stone and oil.Pretreating the stone with a volatile solvent or adding a wetting agentto the oil has been proposed as a partial solution. Ideally, the stoneshould be dry but if it is heated to dry it, much of the economicadvantage of the process is lost. (2) Powdered asphalt, owing tocohesion of particles, electrical charges on the particles, etc., has astrong tendency to clump into aggregates which are not easily dispersedin the oil flux, either when mixed separately or in presence of stoneaggregate. The result is a non-uniform product which produces a poorpavement, soft in some spots and brittle in others. Attempts to correctthis difficulty by more extensive mixing results in reduced output ofthe plant which cannot be tolerated. (3) Applying the oil to stonefirst, then adding asphalt powder tends to localize the powder in areaswhere first brought into contact. Premixing asphalt and oil gives aviscous slurry or partial solution which is difficult to mix with stone,especially when much fines are present, tending to form pasty massesdifiicult to distribute.

We have now discovered that the foregoing problems can be solved by theexpedient of first combining the asphaltpowder and stone aggregatethoroughly mixing and gr nding the two together then adding the flux oilto the mixture and continuing the mixing. By this technique, thepowdered asphalt is dispersed in an amazingly short time as will beshown hereinafter. Dispersion is facilitated by the grinding action ofthe stone as in a ball mill, breaking up all lumps and even furtherreducing particle size of this brittle asphalt. The presence of dust inthe aggregate, for example 2 to 10% passing 200 mesh, combines to aid inthe distribution of the asphalt, partly by coating the particles withrock dust which prevents cohesion and partly by neutralizingelectrostatic charges. As a result all surfaces of the stone are evenlycoated with a gray powder of asphalt and stone dust. When oil is lateradded, this coating of asphalt dust on the stone speeds the wetting bythe oil, thus decreasing the time required to produce a satisfactoryproduct of uniform composition throughout the batch. Even whenconsiderable moisture is present in the stone, e.g.: 1 to 3% by weight,the presence of the rock dust serves to distribute the asphaltparticles, and subsequently the flux oil, over the surface of the stonewithout stripping.

Mixing can be effected in a typical, drum type mixer common to theportland cement industry, the stone and powdered asphalt being chargedand mixed until uniform, then the oil flux added and mixed again untilthe oil is completely dispersed. This operation can be carried out in atransit mixer receiving the weighed charges of aggregate and asp-haltpowder at the stone pile, mixing thoroughly on the way to the job andintroducing the flux oil to complete the mixture before discharging.After the mixture is spread evenly on the roadway or other surface it iscompacted by rolling or beating into the desired form. On ageing, theparticles of asphalt gradually absorb the oil to give a strong, adherentcement holding the stone in place. Vibration from traflic-serves tospeed the hardening process. It is advantageous to spread the mix withinan hour or two from the time of mixing, although this time can beextended to a day or more under favorable conditions such as lowtemperatures, coarser grind of the asphalt and lower asphalt solubilityas in the case of hard blown asphalts and gilsonite. Flux oilcomposition is also important.

We prefer to prepare the asphalt, stone and flux oil mixture in a mixingplant specially designed for the purpose, in which we have provided arapid mixing machine into which the ingredients are introduced in propersequence. Such a plant is shown schematically in the drawing whichaccompanies this application.

Referring to the drawing, aggregate supply hoppers, 10, 11 and 12 arefilled from conveyor 13 and elevator 14 by charging the various gradesof stone into hopper 15 located at or below ground level. Elevator 14 issuitably a bucket elevator of common design and conveyor 13 can beeither belt or helix type. Hydraulically operated clapper valves 16, 17,and 18 regulate the amount of each material discharged into weigh hopper19 equipped with hydraulically operated discharge valve 20.

Asphalt supplied in 50 to 400 lb. blocks is powdered in mill 21,suitably a hammer mill such as the well known Williams or Raymond mill.It is collected in hopper 22 equipped with hydraulically operateddischarge valve 23 which: controls the charge delivered into weighhopper 24 where the desired amount is accurately measured for eachbatch. When the correct weight is indicated by movement of beam 26, anelectronic signal closes the discharge valve-23.

Flux oil from tank 27 flows by line 28 to metering cylinder 29 providedwith piston 30 operated by hydraulic cylinder 31 having an adjustablestroke controlled by screw 32, operated either manually or automaticallyto deliver any desired exact volume of oil thru discharge line 33. Checkvalve 34 prevents oil returning to tank 27 while spring loaded dischargevalve 35 prevents discharge of oil except when ressure is applied bypiston 30. It is desirable to locate valve 35 at the extreme end of thedischarge line 33 to prevent drip when not operating between mixingcycles.

Turbine mixer 36 is preferably of the Smith Turbine type made by T. L.Smith Company of Milwaukee, Wisconsin, wherein plows travel in acircular path, weaving the aggregate rapidly in and out while travellingat a speed of about to feet per second. The charge of aggregate isusually between 1,000 and 4,000 pounds and mixing is completely uniformwithin 60 seconds or less. The cutaway view of the mixer is shown at 37with plow 38 car ried on arm 39 attached to rotating hub 40.

Hydraulically operated discharge plate 41 closes the bottom opening,permitting the mixer to be emptied within less than 5 seconds with themixer running. The finished mix falls through chute 42 into waitingtruck suggested at 43. It is preferred to operate all controlsautomatically by means of a timer which operates the valves in sequencein a manner well known in the art. The mixer 36 may run continuouslywhen the plant is in operation. Following is an example: a truck-trailerrig having a capacity of tons arrives at the plant to receive a load ofmix. A two ton batch of stone, previously weighed into hopper 19 isdropped into the mixer 36 followed immediately by 50 lbs. powdered hardasphalt from 24. After seconds mixing, the stone has a uniform graycolor showing the asphalt particles dispersed evenly throughout. On theinstant, 12 gallons of flux oil is added from line 33 and mixingcontinued 55 seconds. Discharge plate 41 then opens for a period of 5seconds after which the cycle is ilepeated. Time required for loadingthe truck with 20 tons is thus 15 minutes.

The following data illustrate the uniformity of mixing obtained withpowdered asphalt and stone in a Smith Turbine mixer of 2,000 lbs.capacity equipped with six plows travelling at 9 feet per second. Thecharge of stone was made up of 800 lbs. No. 4 inch mesh) and 1,200 lbs.of rock dust.

Screen analysis of the stone is as follows: Percent Passing /i mesh 100Passing 4 mesh 71 Passing 8 mesh 51 Passing 30 mesh 41 Passing 200 mesh(with washing) 7 /2 Screen analysis of the asphalt follows: PercentPassing 50 mesh 90 Passing 100 mesh 55 Passing 200 mesh 20 Second Mix,gm.

60 Second Mix, gm.

Sample 1 Sample 2 These results show asphalt content of 1.625% of themix in which 1.725% asphalt was incorporated. Surpris ingly, they showperfect mixing in 40 seconds, the asphalt concentration being exactlythe same in remote sections of the mixer.

Flux oil in the amount of 100 lbs. was then added and mixing continuedfor 60 seconds. The oil was No. 250 slow curing (8.0.) to which wasadded 5% by volume of an aromatic solvent corresponding to xylene, forthe purpose of reducing viscosity to aid rapid mixing and speedhardening of the concrete on the roadway. When the dough-like product iscompacted by rolling, it can be used for light trafiic immediately andwill support heavy road traffic within three to five days. The xyleneaids hardening on evaporation and by increasing the rate of solution ofthe asphalt particles in the flux oil. Test strips were made byspreading a layer 3 /2 inches thick and rolling to a thickness of 2 /2inches.

Following are typical analyses of slow and rapid curing liquid asphalts(flux oils) suitable for our process:

The hard asphalt employed in our process will have a specific gravity ofabout 1 and softening point of about 250300 F. ball and ring method.Penetration-ASTM at 77 F. may range from 0 to 5. It is very brittle andeasily reduced to powder in an impact mill, care being taken to preventthe mill from heating to a degree where the asphalt particles becometacky. Powdered asphalt is conveniently transferred in an air streamfrom mill to hopper. The amount of asphalt powder employed in ourprocess ranges from about 10 to 35 lbs. per ton of aggregate with fiuxoil proportions of about 75 to 125 lbs. per ton, usually about lbs. or12 gallons. The flux oil dosage varies somewhat with the type of oilused, generally more oil is used with the slow curing type. Addition tothe oil of about 2 to 10% of a volatile aromatic solvent such as xyleneaids in the curing of the pavement.

Following are examples of paving mixtures which have been foundsatisfactory. In each example, one ton of aggregate of the compositiondescribed herein above, was used.

EXAMPLE 1 Lbs.

Rock 2000 Powdered asphalt 12 Flux oilR.C. 250 108 On rolling thepavement, a firm surface resulted which was not affected by rain.

EXAMPLE 2 Lbs. Rock 2000 Powdered asphalt 22 Flux oilblend of R.C. 250with 25% by vol. of

SC. 250 and 5.5% petroleum xylene solvent 108 Rolling twice gave goodcompaction, cured rapidly.

EXAMPLE 3 Lbs. Rock 2000 Powdered asphalt 25 No. 2400 oil 60 No. 800'R.C. oil 40 Three hours after laying pavement, it was hard enough forservice. Next day it was very hard and appeared fully cured.

Although we have described our process with respect to certain limitedapplications by Way of example, We intend its scope be determined onlyby the claims which follow. Uniform dispersion referred to in the claimsmeans that the asphalt content of the mix is the same throughout thebatch, regardless of what area of the batch is analyzed. By rock dust ismeant that part of the stone aggregate passing a standard 200 meshsieve. The term plastic used herein with reference to the paving mixtureis meant to describe the dough-like consistency thereof.

Having thus described our invention, what we claim is:

1. The method of paving with an asphalt and stone mixture withoutheating which comprises thoroughly mixing a stone aggregate withcomminuted hard asphalt to form a uniform dispersion of asphaltparticles on the stone surfaces, then adding a fluxing oil andcontinuing mixing until uniformly dispersed in the aggregate andthereafter applying the plastic mixture to a surface and compacting itthereon to form the desired pavement before solution of asphaltparticles in said fiuxing oil is complete, and said mixture becomesrigid.

2. The method of claim 1 wherein said stone aggregate contains about 2to percent by weight of rock dust which forms a uniform mixture withsaid powdered asphalt during the said initial mixing operation, therebyassisting coating the stone when subsequently contacted with oil.

3. The method of claim 1 wherein the amount of powdered asphalt employedis about 10 to 35 lbs. per ton of stone aggregate.

4. The method of claim 1 wherein the amount of fluxing oil employed isabout 75 to 125 lbs. per ton of stone aggregate.

5. The mehod of claim 1 wherein the said fluxiug oil contains from 2 to10% of a volatile aromatic solvent to aid the solution of asphaltparticles in said oil.

6. The method of claim 1 wherein said stone aggregate is charged to amixing zone simultaneously with said asphalt powder and the rate ofmixing is sufficiently rapid to effect a uniform mixture within aboutseconds and thereafter the fluxing oil is mixed at a rate sufficientlyrapid to effect a uniform mixture in less than about seconds.

References Cited UNITED STATES PATENTS 2,128,291 8/1938 Finley 9423 X2,229,872 1/1941 Pullar 9423 2,349,445 5/1944 McGrane 9423 2,572,06810/1951 Sommer 9423 2,876,686 3/1959 Birney 9423 2,978,351 4/1961 Pullar9423 X 3,074,807 l/1963 Dorius 9423 X NILE C. BYERS, JR., PrimaryExaminer.

